I had a wood one that did something simular, I just used a flat head screwdriver, for eyeglasses, and knocked out the small pieces. I dont recall a ring holding in the screen, I remember it being loose and that I needed to place it on the inner edge of the catcher when I was attaching it.

[----] grinder
1 1 1 /top

[ 1 1 ] center

------- screen

(_____) catcher


yours simular?